udp big packet and MTU

Detect link MTU by send udp big packets

#udp   #mtu  

---

Table of Content

Code demo

udp

#!/usr/bin/env python3
import socket

# Inlined constants, because Python 3.6 has dropped the IN module.
class IN:
    IP_MTU = 14
    IP_MTU_DISCOVER = 10
    IP_PMTUDISC_DO = 2

def send_big_datagram(host, port):
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    # sock.setsockopt(socket.IPPROTO_IP, IN.IP_MTU_DISCOVER, IN.IP_PMTUDISC_DO)
    sock.connect((host, port))
    try:
        sock.send(b'#' * 1500)
    except socket.error as e:
        print(e)
        print('Alas, the datagram did not make it')
        max_mtu = sock.getsockopt(socket.IPPROTO_IP, IN.IP_MTU)
        print('Actual MTU: {}'.format(max_mtu))
    else:
        print('The big datagram was sent!')

if __name__ == '__main__':
    send_big_datagram("172.17.0.2", 1060)

Test

• Create ubuntu docker image with tcpdump and netcat

FROM ubuntu:20.04
RUN apt-get update && apt-get install --no-install-recommends --yes \
    tcpdump \
    net-tools \ 
    netcat
CMD ["/bin/sh"]

Test 1: Test 123

• Run docker image
• Run tcpdump and sniff for icmp
• Send ping from host machine

tcpdump -n -i eth0 icmp

Test 2: send udp packet less then MTU

docker shell1

tcpdump -n -i eth0 udp and port 1060 or icmp

docker shell2

nc -ul 1060

Test 3: send udp packet bigger then MTU

• Run tcpdump and nc like in Test 2
• send with sock.setsockopt(socket.IPPROTO_IP, IN.IP_MTU_DISCOVER, IN.IP_PMTUDISC_DO)
• comment the above line

with IP_PMTUDISC_DO

python examples/python/networking/udp/big_Sender.py
# result
[Errno 90] Message too long
Alas, the datagram did not make it
Actual MTU: 1500

# packet not send at all

without setsockopt

python examples/python/networking/udp/big_Sender.py
# Host Result
The big datagram was sent!
# shell 1: tcpdump
19:11:06.400842 IP 172.17.0.1.47327 > 172.17.0.2.1060: UDP, bad length 1500 > 1472

# shell 2: nc
show 1500 char of '#'

19:14:46.502573 IP 172.17.0.2.1060 > 172.17.0.1.48321: UDP, length 1
19:14:46.502686 IP 172.17.0.1 > 172.17.0.2: ICMP 172.17.0.1 udp port 48321 unreachable, length 37
19:14:51.960248 IP 172.17.0.1.58025 > 172.17.0.2.1060: UDP, bad length 1500 > 1472